Science is terrible at explaining itself to outsiders. There are only two real types of interaction science has with the media and thus the average non-scientist - #1 is some crusty old bloke with a beard using words that sound like he's pulled them straight out of his arse and #2 is some young berk making shit explode and claiming science is thus awesome. You either get your science flung so far over your head there's no hope you'll understand the underlying concept, or you get it so dumbed down that it's not really science.
This means that most of the time, the people who report science to non-scientists aren't scientists themselves and so they get shit wrong, focus on the bad parts and generally screw it all up. Take Trastuzumab/Herceptin, for example. Herceptin is a drug that can be used to treat breast cancer, which is not one of the more dangerous cancers but is one of the more 'fashionable' cancers (compared to say, lung cancer. Most people would rather see breast cancer cured than lung cancer as popular opinion states that lung cancer is caused by smoking and thus lung cancer patients deserve it). There was a bit of an outcry here (the UK) a while back over Herceptin, when it was reported that breast cancer patients were being denied this 'potential miracle cure' because it only works on 25% of the population. Cancer sufferers said they wanted to take the chance. 1 in 4 is good odds, right? The doctors are just worried about their stats and the money it costs, not patients survival, right? Wrong. Herceptin would not work on these patients. They didn't have a 25% chance, they had a 0% chance. Herceptin only works to treat breast cancers that are caused by defects in the HER2 receptor. This is approximately 25% of breast cancers. Therefore, in 25% of cancers, Herceptin works 100% of the time. In 75% of cancers, Herceptin does absolutely nothing. These patients weren't given the drug because there's no point in giving them it. The media reports weren't correct, people in the street didn't understand exactly what the science behind the drug was and suddenly the NHS are scum who let breast cancer metastasise and kill patients who (supposedly) had a chance at survival.
I did have a handful more examples including the LHC and the MMR vaccine stuff, but I don't want to hog the thread.
TL;DR: Science isn't reported properly, so people get the wrong idea.